Performance of technical ceramics

ABSTRACT

Disclosed herein is a ceramic particle comprising a core substrate chosen from yttria-stabilized zirconia, partially stabilized zirconia, zirconium oxide, aluminum nitride, silicon nitride, silicon carbide, and cerium oxide, and a conformal coating of a sintering aid film having a thickness of less than three nanometers and covering the core substrate, and methods for producing the ceramic particle.

RELATED APPLICATIONS

This application is a continuation of U.S. National Stage applicationSer. No. 16/347,585 filed on May 5, 2019, which is the national stageentry of International Application No. PCT/US2017/060069, filed on Nov.4, 2017, and which claims priority to Provisional Application No.62/418,666, filed on Nov. 7, 2016, and Provisional Application No.62/520,665, filed on Jun. 16, 2017, the teachings of both of which areincorporated herein by reference in their entireties.

GOVERNMENT SUPPORT

This invention was made in part with Government support under contractNSF CMMI 1563537 awarded by the National Science Foundation; and GrantAPP-43889 awarded by the State of Colorado Advanced IndustriesAccelerator Program. The Government has certain rights in the invention.

BACKGROUND

Eight mole percent (8 mol %) yttria-stabilized cubic zirconia (“8YSZ”)has practical uses in Solid Oxide Fuel Cells (“SOFCs”), but suffers fromsome inherent shortcomings such as high operating temperature, highsintering temperature, low ionic conductivity, and poor mechanicalstrength. The addition of alumina (Al₂O₃) by ball milling, a processusing high-energy collision of hard balls with a powder mixture of theAl₂O₃ and 8YSZ, has been shown to lower the sintering temperature andincrease mechanical strength and ionic conductivity.

8YSZ is most commonly used in SOFCs as the solid electrolyte because itis a chemically stable and inexpensive option. The SOFC must be operatedat high temperatures, upwards of 700° C., in order to have suitableionic conductivity. This high operating temperature also limits possiblecomponent materials, and requires long start-up times. Further limitingthe application of 8YSZ is the high sintering temperature required tomake dense SOFC electrolytes. Typically, commercial electrolytes aresintered at 1450° C. for about 4 hours. Because an attractive route toSOFC production is co-firing the electrolyte with all other components,for example, anode, cathode, and interconnect, the material requiringthe highest sintering temperature (typically 8YSZ), dictates theco-firing temperature. However, exposure of the non-8YSZ parts to 1450°C. for several hours can have deleterious effects on their performance.There has been an unmet need to minimize both the high sinteringtemperature and the time requirements.

The expectations for the quality and type of products currently producedby 3D printing with available 3D ink have not been met. 3D printing,including Fused Deposition Modeling (FDM) lays down layers of inkmaterial, with the intent that the layers fuse together, forming alaminated 3-dimensional part. 3D printing, including FDM, lays downlayers of ink material, with the intent that the layers fuse together,forming a laminated 3-dimensional part. However, the final parts oroutput from 3D printing have not been consistently good. The final 3Dparts are often fragile, or delaminate easily. The laminate 3D parts maynot bond as well in the Z axis as they do in the X-Y planes, so that aforce from the side may easily fracture the part.

Further, current 3D part printing is generally not good for productionof small parts wherein high resolution is needed. Because the print isin 3 dimensions, resolution depends on the minimum feature size of theX-Y plane, and the Z-axis resolution. Z-axis resolution relates to layerheight, and is less related to print quality. The X-Y resolution, orminimum feature size, is measured via microscopic imaging, and is themore important of the two because it allows for production of finedetail in the parts.

BRIEF SUMMARY OF THE DISCLOSURE

The present invention relates, in part, to a discovery of a ceramicparticle comprising a core substrate chosen from yttria-stabilizedzirconia, partially stabilized zirconia, zirconium oxide, aluminumnitride, silicon nitride, silicon carbide, and cerium oxide; and aconformal coating of a sintering aid film having a thickness of lessthan three nanometers and covering the core substrate. In one embodimentof the invention, the conformal coating of the sintering aid filmcovering the core substrate has a thickness of from less than one (1)nanometer to one (1) nanometer. In another embodiment, the conformalcoating of the sintering aid film has a thickness of about two (2)nanometers.

In yet another embodiment, the conformal coating of the sintering aidfilm is a uniform, conformal coating of the core substrate. In anotherembodiment, the conformal coating of the core substrate includeswell-distributed islands of film across the surface of the ceramicparticles.

Disclosed herein is a ceramic particle wherein the core comprisesyttria-stabilized zirconia or partially stabilized zirconia, and thesintering aid film comprises alumina.

Disclosed herein are methods of forming a ceramic particle comprising acore substrate including a conformal coating of a sintering aid filmhaving a thickness of less than three nanometers, wherein the sinteringaid film covering the core substrate is formed by atomic layerdeposition (“ALD”). In one embodiment of the invention, the ceramicparticle with a conformal coating of a sintering aid film is preparedusing one cycle of atomic layer deposition of the sintering aid film;and then sintered in air at about 1350 degrees Celsius for about two (2)hours. In another embodiment of the invention, the ceramic particle isprepared with from about one cycle to about nine cycles of atomic layerdeposition of a sintering aid.

Also disclosed herein are methods and compositions relating to acolloidal gel or slurry suitable for producing a 3D ink for3-dimensional printing comprising a ceramic particle as disclosedherein, including a core substrate and a conformal coating of asintering aid film having a thickness of less than three nanometers andcovering the core substrate.

Another embodiment of the invention is a solid oxide fuel cellelectrolyte comprising a ceramic particle as disclosed herein, includinga core substrate and a conformal coating of a sintering aid film havinga thickness of less than three nanometers and covering the coresubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 is a graphical representation of Relative Density measuredvolumetrically for different sample types having been sintered in air at1350° C. for 2 hours as a function of the number of ALD cycles, andwherein “BM” represents a Prior Art ball-milled sample.

FIG. 2 is a graphical representation of oxygen ion conductivity atdifferent temperatures in ° C., measured using electrochemical impedancespectroscopy for different sample types having been sintered in air at1350° C. for 2 hours, and wherein “BM” represents a Prior Artball-milled sample.

FIG. 3A is a graphical representation of the relative density (%theoretical) as a function of temperature during constant rate ofheating at 10° C./min heating rate, in dilatometer experiments for allsamples analyzed (coated and uncoated).

FIG. 3B is a graphical representation of the relative density (%theoretical) as a function of temperature during constant rate ofheating at 15° C./min heating rate, in dilatometer experiments for allsamples analyzed (coated and uncoated).

FIG. 3C is a graphical representation of the densification rate (1/K) asa function of temperature during constant rate of heating at 10° C./minheating rate, in dilatometer experiments for all samples analyzed(coated and uncoated).

FIG. 3D is a graphical representation of the densification rate (1/K) asa function of temperature during constant rate of heating at 15° C./minheating rate, in dilatometer experiments for all samples analyzed(coated and uncoated).

FIG. 4 is a graphical representation of the apparent activation energyof densification as a function of number of ALD cycles, wherein theactivation energy was determined from a series of constant rate ofheating dilatometer experiments.

FIG. 5 is a graphical representation of the decrease in ionicconductivity (S/cm) when decreasing the sintering temperature from 1450°C. to 1350° C. as a function of measurement temperature and number ofALD cycles, wherein conductivity was measured using electrochemicalimpedance spectroscopy.

FIG. 6 is a bar graph showing, for zero to 5 ALD cycles, the increase inR GB/R bulk at 300° C. defined as (the ratio of grain boundaryresistivity to bulk resistivity after sintering at 1450° C. for 2 h)minus (the ratio of grain boundary resistivity to bulk resistivity aftersintering at 1350° C. for 2 h) as measured using electrochemicalimpedance spectroscopy at 300° C. in air.

DETAILED DESCRIPTION

The invention inter alia also includes the following exemplaryembodiments, alone or in combination. It will be understood that theparticular embodiments of the invention are shown by way of illustrationand not as limitations of the invention. At the outset, the invention isdescribed in its broadest overall aspects, with a more detaileddescription following.

Disclosed herein are a process comprising adding a thin film of ceramicsintering aid of desired thickness (wt %) to a primary ceramic powder byatomic layer deposition using, for example, an agitated powder reactor,and a product formed by the process. For use in preparing a 3D ink,e.g., incorporation of a conformal coating of the sintering aid aroundeach primary ceramic substrate particle improved the fabricated partproperties associated with grain boundary phenomena such as impurityscavenging, grain boundary diffusion, grain growth, liquid-phasesintering, ionic conductivity, thermal conductivity, etc., and increasedthe homogeneity of dense parts compared with conventional techniquessuch as ball milling, spray drying, or sol-gel processing. Sintering isthe process of causing a material to become a coherent or compact, densemass by applying heat and/or pressure without melting or liquefying thematerial. As the terms are used herein, “densification” and “sintering,”and grammatical variations thereof, have the same meaning. A sinteringaid helps impart integrity and compressive strength to the materialbeing sintered.

Additionally, in the preparation of a ceramic slurry or colloid in, forexample, tape casting or additive manufacturing via direct ink writing,the thin film of sintering aid enables facile rheology control byexposing only one surface (the sintering aid), as compared toconventional sintering aid incorporation methods wherein multiplesurfaces, and thus multiple surface charges will be present (i.e., thesintering aid and the primary ceramic). For fine ceramic precursorpowders that are prone to degradation in aqueous environments, e.g.,aluminum nitride (AlN), the thin and pinhole-free conformal coating ofan oxide ceramic sintering aid, renders the particles resistant todegradation by water, thereby enabling aqueous processing which iscritical in, for example, direct ink writing (additive manufacturing). 3wt % yttrium oxide is a good sintering agent for aluminum nitride. Amore detailed description follows.

Atomic Layer Deposition (ALD), a thin film deposition technique, is aself-limiting surface reaction that deposits uniform layers of thedesired precursor onto the particle surface. This is done by fluidizingthe particles and adding two different precursors in sequence such thatthere are two reactions occurring sequentially. For the addition ofAl₂O₃ to 8YSZ, the reaction utilizes trimethylaluminum (TMA) and wateras precursors, adding first TMA, then water, then TMA, and so on. Oneaddition of TMA plus one addition of water comprises one cycle. Thereaction proceeds in a fluidized bed reactor to help ensure coating ofall surfaces.

Previous studies have investigated the effects of Al₂O₃ on the kineticsof 8YSZ densification. In these studies, undoped 8YSZ was compared withsamples mixed with Al₂O₃ concentrations ranging from 0.1-10 wt % Al₂O₃where Al₂O₃ was incorporated using a conventional process such asmilling, spray drying, or sol-gel-type processing with the optimalamount for minimizing the sintering temperature and apparent activationenergy typically found to be less than (<) 1 wt %.

In accordance with the present invention there is provided a methodwherein atomic layer deposition (ALD) was used to add Al₂O₃ to 8YSZ as asintering aid. The effects of the ALD-produced mixture on sinteringbehavior, kinetics, and ionic conductivity, and for comparison to 8YSZas purchased were tested. Al₂O₃ was deposited as a conformal coating on8YSZ particles at concentrations ranging from approximately (˜) 1 toapproximately 4 wt %, corresponding with 1 to 9 ALD cycles,respectively. According to another embodiment of the invention, the massor weight of the alumina in the sintering aid film is from about 0.2weight percent to about 2 weight percent of the ceramic particle. Forall samples, the addition of Al₂O₃ reduced the temperature required forsintering by ˜100° C. and decreases the apparent activation energy ofdensification. The optimal concentration of Al₂O₃ was found to be about2.2 wt % corresponding with about 5 ALD cycles which reduced theapparent activation energy from ˜700 kJ/mol to ˜400 kJ/mol. A ceramicparticle prepared according to an embodiment of the invention isnon-reactive with water.

Three-Dimensional (3D) Printing Ink

There has been an ongoing need in the 3D ink industry for a method ofachieving a more uniform distribution of ceramic and sintering aid,increased reliability and consistency of parts produced, and increaseddensification with closer packing of particles, which aspects arecritical for the production of small parts.

We investigated the effects of the addition of Al₂O₃ by ALD ondensification and ionic conductivity, developed a strong 3D inkformulation, and successfully printed 3D lattice structures to befurther analyzed. With the addition of different amounts of Al₂O₃ to8YSZ, the effects on densification behavior were examined throughconstant rate of heating experiments using a horizontal push-roddilatometer. The effects examined included the sintering temperature,the temperature at which the maximum densification rate occurs, thekinetics by which the densification proceeds, the overall ionicconductivity as a function of temperature, and the relativecontributions of grain boundary and bulk resistivity. The inkformulation was determined by preparing numerous batches of ink andvarying the solids loading and relative polymer amounts in order toobtain the desired rheology for printing with maximum solids vol %. Theoptimized ink was then used to print 3D structures that were sinteredand examined under the SEM.

One embodiment of the invention is a material composition for 3Dprinting, the composition comprising dispersed solids in a colloid, thesolids coated conformally with a solid sintering additive. In anotherembodiment, the dispersed solids are coated conformally with the solidsintering additive by atomic layer deposition (ALD). In yet anotherembodiment, the conformal coating is also uniform throughout. Theinvention inter alia includes the following, alone or in combination.

Prior art ball milling of Al₂O₃ and 8YSZ yields only a reasonablyuniform distribution of ceramic and sintering aid. However, in contrastto ball milling, using ALD coating of particles yields precise, uniform,conformal coating of 8YSZ ceramic particles with Al₂O₃. In coating everysubstrate particle uniformly by ALD, one can ensure that the sinteringaid is utilized throughout the densifying matrix. Uniform, conformalcoating also allows use of a lower temperature for densification, whichwill also reduce the tendency for grain sizes/flaw sizes to grow. Thelower temperature also appears to reduce the amount of sintering aidneeded to be deposited on the substrate particles in order to achievehigh density upon part fabrication.

With prior art methods, it is expected that the incorporation of thesintering aid as a particle additive will result in particulateinclusions at triple grain junctions in the densified matrix. Incontrast, when the sintering aid additive is deposited according to anembodiment of our invention as a conformal coating or a uniform,conformal coating by ALD, the additive will exist as an intergranularamorphous thin film, instead of as particulates. The presence of anintergranular, amorphous, uniform, conformal film coating of the ceramicparticles enables lower temperature densification and improvedhomogeneity of dense parts.

Stability is critical for use in 3D printing, or additive manufacturing,and a colloidal gel needs to be prepared from the core/shellsubstrate/sintering aid particles. Since colloidal properties are theresult of fine tuning the dispersion in order to suspend particles inthe ink/gel, it is critical to optimize the chemical characteristics ofthe suspended ceramic particles. For conventional ball-milledprecursors, there will be two surfaces (the substrate ceramic and thesintering aid) that require stabilization, and the gel formulation willbe a compromise of properties for the substrate and sintering aid. ForParticle ALD coatings, there is only one surface to be optimized, thatof the sintering aid which surrounds each particle. Hence, it is notonly easier to prepare colloidal gels, but also easier to prepare gelsthat are truly optimized for the system. This improves the preparationof 3D inks/gels having improved flowability for additive manufacturingand ultimately part-to-part reliability.

Importantly, using ALD rather than ball milling, one can use substrateceramic particles of smaller size, and therefore achieve closer packingand overall improved uniformity of densification. This unexpected resultdemonstrates the criticality of using ALD to conformally coat substrateceramic particles in producing 3D ink, and is important for productionof very small 3D parts, for which full densification is required.

Solid Oxide Fuel Cells

At least because the ionic resistance of 8YSZ increases with decreasingtemperature, the costs of running SOFCs currently remain high becausethey must be run at high temperatures. Therefore, there is an unmet needof being able to lower the operating temperature of the fuel cell, whichcan be done by improving the properties of 8YSZ. 8YSZ has a highsintering temperature of 1450° C. However, the addition of Al₂O₃ hasbeen demonstrated to lower this temperature, similarly reducing thecosts of manufacturing SOFCs. It has also shown positive effects onimproving ionic conductivity at lower temperatures which would in turnlower the operating temperature of the SOFC. It may be important thationic conductivity of 8YSZ remain fairly stable when used in SOFCs.

Exemplification

The primary equipment used for the dilatometer studies included thedilatometer, planetary centrifugal mixer, hydraulic press, andcylindrical steel die. The dilatometer requires ceramic powder compactsthat are cylindrical in shape. To achieve this, 8YSZ powder, both withand without Al₂O₃, were mixed with a polymeric binder, which was asolution of 2 wt % poly(vinyl alcohol) (Acros Organics, 98.0-98.8%hydrolyzed, average molecular weight ˜31,000-50,000 grams/mole) and 98wt % deionized water. The binder was incorporated by mixing 4 grams ofbinder, 20 grams of 8YSZ powder, and 25 grams of grinding media (TosohCorp., 5 mm diameter, YTZ Grinding Media) in a planetary centrifugalmixer for 30 seconds at 1100 rpm. The planetary centrifugal mixerprovides sufficient dispersion of the binder throughout the ceramicpowder by simultaneously undergoing high speed revolution and rotation,while the grinding media aids in dispersion.

After sufficient mixing, the 8YSZ powder/binder mixture was separatedfrom the milling media. Cylindrical compacts were made by placing 0.45grams of 8YSZ powder/binder mixture into a 6 mm inner diametercylindrical die, which was exposed to 1 ton of pressure for 90 secondsusing a hydraulic press. The compact was then ejected from the die andassigned a compact number. Samples were pressed for 8YSZ powder with 0-9Al₂O₃ ALD cycles.

The dilatometer comprises a sample holder, furnace, and push rod tomeasure displacement. A constant force of 35 centinewton (35 cN) wasexerted on the push rod to maintain constant contact with the samplebeing tested as it shrinks during heating.

Each sample was sintered in air. Before the compact was inserted intothe dilatometer, the length was measured and recorded using calipers.The compact number was recorded along with the heating test that was tobe run. Constant rate of heating experiments were then carried out asfollows: heating rate of 1° C./min from room temperature to 600° C.(binder burnout), desired heating rate (5, 10, 15, or 20° C./min) from600° C. to 1550° C., cooling back to room temperature at 20° C./min. Thelinear shrinkage was recorded during these experiments by thedilatometer which was calibrated using a sapphire standard. Theshrinkage was then related to density by correcting for the thermalexpansion of the samples using the cooling portion of the curves andassuming isotropic shrinkage. As such, these experiments enable thedensity to be produced as a function of temperature, heating rate, andsample type. The densification rate is then taken as the firstderivative of the density with respect to temperature, approximatedusing finite difference. The density and densification rate can then beused to approximate the apparent activation energy for densification byutilizing an Arrhenius-type dependence of the rate of densification ontemperature.

The principal equipment for ink formulation is a high precision scaleaccurate to 0.1 milligrams and a planetary centrifugal mixer. Thecentrifugal planetary mixer allows thorough mixing of high viscosityinks. It works by orienting the container at 45° relative to thevertical axis and spinning the container counterclockwise. As thecontainer spins counterclockwise on its own axis, the container is spunalong the vertical axis in a clockwise direction which causes verticalspiral convection and exerts a force of 400 G on the ink, effectivelyevacuating all air as well. The ink formulation begins with a smallcontainer fitted for the planetary mixer, which is where the ink will bemade and stored. First, YTZ grinding media is added to aid in mixing.11.56 grams of water was added, then 3.86 grams of ammonium polyacrylate(Darvan 821A) as dispersant. The mass of powder was added in two partsto help ensure uniform mixing. First, 37 grams of 8YSZ powder (coated oruncoated) was added to the aqueous solution which was then placed in theplanetary mixer and set to mix for 30 seconds at 1100 rpm. Another 37grams of 8YSZ powder was added to total 74 grams of powder. It was mixedagain for the same time and speed. Next, 1.08 grams of hydroxypropylmethylcellulose (HPMC) were added as a viscosifier to prevent separationof the ink into component parts. The colloidal ink was mixed once more,then the sides were scraped down and it was mixed again. The finalingredient addition is polyethylene imine (PEI) which is in a 40 wt %solution with water to lower viscosity and allow handling. One drop wasadded, or about 0.02 grams, as a flocculant. The ink was mixed one lasttime with a final solids volume of 43.5%. The ink was then sealed fromair until use.

The principal equipment for direct ink writing and sintering of partsincludes a 3D printer, oil bath, and high temperature furnace. So thatit may operate with very high precision, the 3D printer utilizes magnetsto move. The printer was connected to a computer and operated from thereusing a specifically designed program. First, a syringe was filled withthe prepared 3D ink, and fitted with a tip having a diameter of 330microns. The oil bath was placed underneath the 3D printer, and theprinting substrate was placed in the oil bath. The substrate was aceramic, and dark in color to allow for visualization of the white ink.To begin printing, the syringe was placed in the printer, and theprinter was lowered until just touching the surface of the substrate;then it was raised 200 microns. Rasters, or series of parallel scanninglines, were initiated in order to get the ink flowing smoothly beforebeginning the print job. Using the computer program, the desired shapewas selected and once ready, the printer automatically carried out theprint job. Once it completed printing, the piece was removed from theoil bath and left to dry for 48 hours in air. The pieces were sinteredin a high temperature tube furnace. The sintering process began withbinder burnout, then heated to 1450° C. at a rate of 1.5° C./min; thenheld at the maximum temperature for 1 hour. It was then cooled at a rateof 20° C./min to room temperature.

The principal equipment for ionic conductivity measurements are amechanical press, a sintering furnace, and an electrochemical impedancespectrometer. 8YSZ powder (coated or uncoated) were mixed with 2-3 dropspoly(vinyl alcohol) and pressed to a thickness of ˜0.5″ by a mechanicalpress. Pressed pellets were then densified in air at either 1350° C. or1450° C. for 2 h. The sintered pellets were then painted with aconductive platinum paste and inserted into a furnace for theelectrochemical impedance spectrometry measurements. Ionic conductivitywas measured in air at temperatures ranging from 300-800° C.

Results and Discussion

Al₂O₃ was deposited on commercial 8YSZ powder by means of ALD. The ALDprocess exhibited a nearly linear growth rate with number of cyclesenabling the deposition of Al₂O₃ at a controllable concentration. TheAl₂O₃ was precisely deposited as a uniform and conformal coatingcovering each primary 8YSZ particle as a thin amorphous film. Thepresence of Al₂O₃ by ALD enables pellets to reach near theoreticaldensity (>94%) after sintering in air for 2 h at 1350° C. This samedensity is not reached for either the YSZ with no Al₂O₃ or the YSZ withAl₂O₃ incorporated by ball milling as seen in FIG. 1 .

The precise incorporation of Al₂O₃ by ALD decreased the temperaturerequired to sinter/densify by ˜100° C. for all Al₂O₃ concentrationsinvestigated. FIG. 3A depicts the relative density (% theoretical) as afunction of temperature during constant rate of heating at 10° C./minheating rate. FIG. 3B depicts the relative density (% theoretical) as afunction of temperature during constant rate of heating at 15° C./minheating rate. For both heating rates, the uncoated samples had lessrelative density than did the coated samples. Similarly, thedensification rate in the initial stage of sintering (relative density<80% theoretical) was found to be greater for all coated samples thanthe uncoated samples at all temperatures within this regime (FIG. 3C andFIG. 3D.). The temperature at which the maximum densification rate isobtained is similarly decreased by ˜100° C. for all coated samples whencompared to the uncoated 8YSZ except the 9 ALD sample for which thetemperature is decreased by <100° C.

An Arrhenius-type analysis of the densification rate as a function oftemperature over densities within the initial stage (60-80% density)reveals the apparent activation energy of densification for each sampletype. The activation energy is the highest for the uncoated sample,decreases slightly at low Al₂O₃ concentrations (1, 3 cycles) and highAl₂O₃ concentrations (7, 9 cycles), and decreases significantly at theoptimal concentration (of those evaluated) of 5 ALD cycles or ˜2.2 wt%/o Al₂O₃(FIG. 4 ). This dramatic change in activation energy at the 5ALD incorporation level suggests that a conformal ALD film of thisthickness (˜0.5-0.7 nm) enables a low activation energy diffusionprocess to occur. It is expected that from about 1 to about 3 ALDcycles, a monolayer does not exist around each 8YSZ particle, insteadpreferring the formation of small Al₂O₃ islands forming a submonolayerof coverage. At 5 ALD cycles, we did have a conformal monolayer of Al₂O₃around each substrate particle, ˜0.5 nm in thickness. As the number ofALD cycles is increased, this monolayer grows in thickness to ˜1-1.4 nmat 9 ALD cycles. The minimum activation energy is found for the 5 ALDfilm of ˜0.5 nm thickness, suggesting that this is the optimal thicknessfor an intergranular amorphous film to be thick enough to dissolvesufficient cations (Zr⁴⁺) but thin enough to enable facile diffusionfrom grain to grain. At lower thicknesses, the intergranular diffusionpath will be insufficiently formed. At higher thicknesses, the film willbe sufficiently thick to act in part as a barrier to intergranulardiffusion.

A reduction in sintering temperature is expected to have deleteriouseffects on the ionic conductivity of 8YSZ electrolytes due in part tothe retention of pores or defects in the microstructure. Ionicconductivity measurements were obtained for 8YSZ (coated and uncoated)using electrochemical impedance spectrometry following two sinteringprocedures −1450° C. for 2 h and 1350° C. for 2 h. The decrease inconductivity accompanying the decrease in sintering temperature wasfound to be diminished for all coated samples evaluated (1-7 ALDcycles). The conductivity decrease is similarly expected to bemeasurement temperature-dependent. However, we found the ionicconductivity decrease to be approximately constant as a function ofimpedance temperature, with the exception of the 3 ALD sample. As such,the Al₂O₃ ALD coatings demonstrate that a reduction in sinteringtemperature is not accompanied by a reduction in ionic conductivity(electrolyte performance) as is the case in the uncoated sample. Theionic conductivity of samples sintered at 1350° C. for 2 h was found tobe optimized or maximized with 1 ALD cycle of Al₂O₃ (0.7 wt %) as seenin FIG. 5 . The performance of this sample was found to be superior toYSZ with no Al₂O₃ and YSZ with Al₂O₃ incorporated by ball milling. Ionicconductivity decrease is defined as (conductivity after sintering at1450 C minus conductivity after sintering at 1350° C.). Coated samplesoutperformed the uncoated sample at all temperatures. The benefit of theALD coatings increases with temperature.

Low temperature (300° C.) electrochemical impedance spectrometry can beused to decouple the relative contributions of the grain boundaries andthe grain interior to the total resistivity of the electrolyte.Following sintering at two temperatures as described previously, we notethe increase in grain boundary resistivity is significant for theuncoated sample but less so for the coated samples, particularly for the5 ALD sample (FIG. 6 ). This suggests that the ALD coatings sufficientlyalter the microstructure, and particularly the grain boundarymicrostructure, such that resistivity at the grain boundary is reducedfollowing reduced temperature sintering compared with the uncoatedsample.

Colloidal gel ink formulations were developed for 8YSZ with 0, 1, and 3Al₂O₃ ALD cycles. For 8YSZ with 0 Al₂O₃ ALD cycles, the optimum solidsvolume percentage was found to be between 43.5 vol % to just under 44vol %. A printable 8YSZ ink can be made with 44 vol % solids, but it isprone to thickening with time, causing the printer to clog and stall,rendering the printed part unusable. However, inks consisting of 43.5vol % solids could be printed reliably, and had a high viscosity toresist deformation and retain their shape after extrusion. Additionally,it was shown that the ink formation with 43.5 vol % solids contentexperiences minimal warping and uniform shrinkage after densification.At a solids loading greater than 44 vol %, the colloidal ink collapsesand becomes unprintable and firm. The polyelectrolyte, Darvan, no longereffectively disperses the mixture and phases of liquid and powder beginto separate. Solids loading less than 43 vol % leads to deformationafter extrusion, and an increased likelihood of warping and crackingduring drying and densification. The final optimized ink formulation for8YSZ with 0 Al₂O₃ cycles was 43.5 vol % 8YSZ powder, 41.5 vol % water,11.1 vol % Darvan, 3.8 vol % hydroxypropyl methylcellulose, and 0.2 vol% PEI.

The optimized ink formulation for 8YSZ was then extended to 8YSZ with 1and 3 Al₂O₃ ALD cycles. The ink formulation for 8YSZ with 1 Al₂O₃ ALDcycle was 42.4 vol % 8YSZ/Al₂O₃ powder, 42.0 vol % water, 11.4 vol %Darvan, 4.1 vol % hydroxypropyl methylcellulose, and 0.2 vol % PEI.Additionally, the ink formulation for 8YSZ with 3 Al₂O₃ ALD cycles was39.4 vol % 8YSZ/Al₂O₃ powder, 44.8 vol % water, 12.2 vol % Darvan, 3.4vol % hydroxypropyl methylcellulose, and 0.2 vol % PEI. Both theseformulas led to the successful fabrication of 3D square latticestructures using direct ink writing, where clogging did not occur duringprinting and final parts did not warp or deform.

The rheology of the ink chosen is highly dependent on the surfacechemistry of the ceramic particles. A solution of 8YSZ and water has apH of approximately 7 and, with a basic isoelectric point, the 8YSZsurface becomes positively charged. Van der Waals forces cause the 8YSZparticles to agglomerate, so the negatively changed polyelectrolyte,Darvan, was added to homogeneously disperse the 8YSZ powder throughelectrosteric repulsion. The dispersant allows for ink homogeneity, buta flocculant must be added to ensure the ink is stronger and hasdesirable mechanical properties to resist deformation. The flocculantadded was PEI, which is a positively charged polyelectrolyte. A smallamount was added so that some, but not all, of the dispersant effectswere countered. Viscosity was adjusted by adding hydroxypropylmethylcellulose, and ensured that the ink would not separate out intoindividual components. The final result was a homogenous, viscous inkthat prints without separating and holds it shape during extrusion,drying, and densification.

It was found that the ink formulations for 8YSZ with 0, 1, and 3 Al₂O₃ALD cycles required similar amount of dispersant and flocculant, onlyvarying slightly in water and Darvan content, to control the particlesurface chemistry and produce an ink with required rheology. The inkformulation is dependent on the particle surface chemistry, and at 1 and3 Al₂O₃ ALD cycles, only a sub-monolayer of Al₂O₃ is present. Theparticle surface consists of both 8YSZ and Al₂O₃. Therefore, it can beseen that the particle surface chemistry of both 8YSZ and 8YSZ with 1and 3 Al₂O₃ cycles respond similarly to the polyelectrolytes utilized inthe production of the colloidal gel ink.

It was also found that a significant amount of shrinkage occurs duringdensification for 3D printed colloidal gel inks. For 8YSZ with 0 Al₂O₃ALD cycles, the initial dimensions of all printed parts areapproximately 35.3 mm by 35.5 mm. After drying the dimensions are 34.7mm by 34.8 mm, which is a shrinkage of about 4%. The dimensions of thesintered pieces on average are 27.3 mm by 26.9 mm, which is a shrinkageof about 40%. This is due to the elimination of water, hydroxypropylmethylcellulose, Darvan, and PEI from the part and the reduction ofpores from between ceramic grains.

According to an embodiment of the invention, a ceramic particle has aconformal coating of the sintering aid film covering the core substrate,and is formed by atomic layer deposition using a system chosen from afluid bed reactor, a vibrating reactor, a rotating reactor, a spatialsystem wherein precursor gases are separated in space, and a batchreactor, and any desired combinations thereof.

A ceramic particle according to an embodiment of the invention has acore comprising cerium oxide and a sintering aid film chosen fromalumina, titanium oxide, yttrium oxide, calcium oxide, iron oxide,copper oxide, chromium oxide, boron oxide, silicon dioxide, nickeloxide, and any desired combinations thereof. In another embodiment, thecore of the ceramic particle comprises aluminum nitride, and thesintering aid film is chosen from yttrium oxide, magnesium oxide,calcium oxide, silicon dioxide, lanthanum oxide, and any desiredcombinations thereof.

A ceramic particle according to an embodiment of the invention has acore chosen from silicon nitride and silicon carbide, and the sinteringaid film is chosen from yttrium oxide, alumina, magnesium oxide,lutetium oxide, ytterbium oxide, and any desired combinations thereof.

Analysis and Recommendations

The precise coating of 8YSZ with Al₂O₃ by ALD is effective in reducingthe sintering temperature and temperature at which the maximum rate ofdensification is obtained by ˜100° C. The apparent activation energy fordensification was found to similarly decrease for all coated sampleswhen compared with the uncoated 8YSZ. The optimal level of ALDincorporation for maximally reducing the apparent activation energy wasfound to be 5 ALD cycles or ˜2.2 wt % Al₂O₃.

The precise conformal coating of 8YSZ with Al₂O₃ by ALD is similarlyeffective in enabling sufficiently conductive electrolytes whenutilizing a reduced sintering temperature. For this application, 1 ALDcycle or ˜0.7 wt % Al₂O₃ was found to be the optimal, although 5 and 7ALD cycles (˜2.2 and ˜3.3 wt %) perform similarly. At low temperature(300° C.), the resistivity of the grain boundaries is decreasedsubstantially after sintering at low temperature for the coated samples,particularly the sample with 5 ALD cycles.

For uncoated 8YSZ powder, a colloidal gel ink formulation with maximumsolids loading was determined to reliably produce 8YSZ ceramic partsthat did not warp or deform during sintering. The colloidal gel inkformulation for 8YSZ powder was then modified for 8YSZ powder with anAl₂O₃ coating by 1 and 3 Al₂O₃ ALD cycles, and the formulation wasdetermined to reliably print ceramic parts from core/shell 8YSZ/Al₂O₃powder that did not warp or deform during sintering. The addition of anAl₂O₃ coating to 8YSZ powder reduces the sintering temperature incomparison to uncoated 8YSZ powder. That is, one can print andsinter/densify uncoated 8YSZ powder, but it would require higherdensification temperatures to do so than is required for parts producedfrom 8YSZ powder with a coating from 1 and 3 Al₂O₃ ALD cycles.

3YSZ represents zirconia doped with three mole percent (3 mol %) yttria.3YSZ is also referred to as “Y-TZP.” The colloidal gel ink formulationfor 3YSZ (partially stabilized zirconia or Y-TZP), conformally coatedwith alumina by atomic layer deposition, was adjusted and optimized. ALDcoating of zirconium ceramic particles with alumina should be beneficialas a sintering aid for any level of yttrium doping of zirconium oxide.In practice, we found that the amount of doping can be varied slightlyfrom about 3 percent to about 8 percent in order to obtain differentproperties.

The dopant concentration in zirconia dictates the crystal structure ofthe material. Zirconia doped with three mole percent, 3YSZ, is themechanically strong tetragonal phase, and has been used in dentalceramics. We herein disclose that when 20 wt % Al₂O₃ is added by ALD asa conformal coating to 3YSZ (ATZ) the mechanical properties are furtherenhanced. Alumina toughened zirconia (ATZ) ceramics, wherein the aluminahas been added as a conformal coating by ALD, could be importantmaterials for use in biomedical implants and other engineeringapplications requiring high strength and abrasion resistance at ambienttemperature.

As the terms are used herein, the number preceding the “YSZ” indicatesthe molar percentage doping by yttria. 8 mol % doping optimallystabilizes the cubic crystal structure of ZrO2 which is preferred foroxygen ion conduction (e.g., in a solid electrochemical device). 8YSZ iscommonly referred to as “yttria-stabilized zirconia,” “yttria-stabilizedcubic zirconia,” “cubic stabilized zirconia,” or “fully stabilizedzirconia.” 3YSZ can also be referred to as “yttria-stabilized zirconia,”but is more commonly referred to “tetragonal zirconia polycrystal,”“TZP,” “Y-TZP,” “tetragonal polycrystalline zirconia”,“yttria-stabilized tetragonal zirconia”, or “partially stabilizedzirconia.”

Other “stabilizers” such as Sc₂O₃ can also be used to control thecrystal structure of ZrO₂ in the same manner as Y₂O₃. In yet otherembodiments of the invention, the Al₂O₃ sintering aid deposited byatomic layer deposition should be beneficial also in these cases.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith. All of the features disclosed in this specification(including the accompanying drawings), and/or all of the steps of anymethod or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive. The drawings in the Figures are not necessarilyto scale. The invention is not restricted to the details of anyforegoing embodiments. The invention extends to any novel one, or anynovel combination, of the features disclosed in this specification(including any accompanying drawings), or to any novel one, or any novelcombination, of the steps of any method or process so disclosed.

EQUIVALENTS

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the invention.

What is claimed is:
 1. A colloidal gel comprising: ceramic particlescomprising a core substrate chosen from yttria-stabilized zirconia,partially stabilized zirconia or cerium oxide, and a coating of asintering aid film comprising alumina and having a thickness of lessthan three nanometers and covering the core substrate and wherein thecoating is prepared with 1 to 9 cycles of atomic layer deposition;water; and a viscosity adjusting agent.
 2. The colloidal gel of claim 1,wherein the coating of the sintering aid film covering the coresubstrate has a thickness of from less than one nanometer to onenanometer.
 3. The colloidal gel of claim 1, wherein the atomic layerdeposition uses a system chosen from a fluid bed reactor, a vibratingreactor, a rotating reactor, a spatial system wherein precursor gasesare separated in space, or a batch reactor.
 4. The colloidal gel ofclaim 1, wherein the core substrate is yttria-stabilized zirconia andthe sintering aid film coating comprises approximately 2.2 wt % aluminaadded by atomic layer deposition wherein wt % refers to wt % of theceramic particle.
 5. The colloidal gel of claim 1, wherein the coresubstrate is partially stabilized zirconia.
 6. The colloidal gel ofclaim 1, wherein the core substrate is cerium oxide.
 7. The colloidalgel of claim 1, wherein the ceramic particles are prepared with onecycle of atomic layer deposition and wherein the coating of a sinteringaid film has a thickness of less than one monolayer.
 8. The colloidalgel of claim 1, wherein the core substrate is yttria-stabilizedzirconia, and an yttrium oxide doping of the yttria-stabilized zirconiais about 8 mol %.
 9. The colloidal gel of claim 1, wherein the coresubstrate is partially stabilized zirconia and an yttrium oxide dopingof the partially stabilized zirconia is about 3 mol %.
 10. The colloidalgel of claim 1, wherein the core substrate is partially stabilizedzirconia and an yttrium oxide doping of the partially stabilizedzirconia is about 4 mol %.
 11. The colloidal gel of claim 1, wherein amass or a weight of the alumina in the sintering aid film is from about0.2 wt % to about 2 wt % of the ceramic particle.
 12. The colloidal gelof claim 1, wherein the coating of the sintering aid film covering thecore substrate comprises islands of film across the surface of theceramic particle.
 13. A colloidal gel ink comprising the ceramicparticle of claim 1, wherein the core substrate is yttria-stabilizedzirconia or 8YSZ, and wherein the colloidal gel ink comprises 42.4volume % to 44.8 volume percent solids.
 14. The colloidal gel of claim1, further comprising a dispersant.
 15. The colloidal gel of claim 1,further comprising a flocculant.
 16. A solid oxide fuel cell electrolytecomprising the ceramic particle of claim 1, wherein the ceramicparticles are sintered.
 17. The solid oxide fuel cell electrolyte ofclaim 16, wherein the coating of the sintering aid film is a uniform,conformal coating of the core substrate.
 18. A solid oxide fuel cellelectrolyte made by sintering the particles of claim
 1. 19. The solidoxide fuel cell electrolyte of claim 18 wherein the coating of thesintering aid film is prepared with one cycle of atomic layer depositionand sintered in air at about 1350° C. for about 2 hours.